1. Field of the Invention
This invention relates generally to the field of crosslinked polyimides. More particularly, it relates to crosslinked aromatic polyimides having shape memory properties at elevated temperatures and methods of making the same.
2. Description of the Related Art
Shape memory materials, including shape memory polymers (SMPs) and shape memory alloys (SMAs) are a class of active materials can be programmed to “fix” a temporary shape or a series of temporary shapes and to recover to a “memorized” permanent shape upon application of a predetermined external stimulus. The permanent shape of most SMPs is established during the manufacturing process by a network of covalent or physical crosslinking. While the shape memory effects of SMAs stem from martensitic/austenitic transitions (changes in crystal structure), the shape memory effect of thermally-induced SMPs is driven by heating the polymer above its glass transition temperature (Tg) or melting point (Tm), which causes the SMP to become soft and elastomeric in nature. The heated SMP may be deformed into one or more temporary shapes. The SMP is then cooled below the Tg or Tm while still under stress, causing immobilization of the constituent network chains to fix the temporary shape. Recovery of the permanent shape is then accomplished by heating the SMP above the Tg or Tm, which remobilizes the network chains and allows rubber (entropic) elasticity to return the SMP to its equilibrium or permanent shape. Other types of SMPs include light-induced, electro-active, pH-responsive, and water/moisture-driven SMPs.
SMPs and SMAs have been widely used in actuation, robotics, and piping, as components in aircraft and automobiles, and in medical and dental applications. SMPs possess many properties that make them more attractive than SMAs, such as much lower cost, easier manufacturing and processing using conventional methods, higher capacities for elastic deformation (up to 200% in most cases), lower density, and a broader range of customizable application temperatures. In addition, many SMPs have the potential for biocompatibility and biodegradability. However, most currently available SMPs consist of high-alkyl content polymers such as polyurethane, poly(ε-caprolactone), poly(norbornene), (ethylene-oxide)/(ethylene terephthalate)-based copolymers, styrene/butadiene copolymers, thiolene/acrylate copolymers, etc. Many of these SMPs do not possess shape memory properties above 150° C., nor do they possess long-term thermal and thermo-oxidative stability in this temperature region.